Inductively coupled plasma (ICP) sources have been widely used in atomic emission spectrometry and mass spectrometry [1–2]. Most work in atomic absorption spectrometry (AAS) utilizes flames and electrothermal devices as atomizers for AAS measurement [3,4], although the use of plasma sources as atomization cells for AAS measurement was proposed in the early development stage of plasma spectrometry. In the first report, a multiple beam system for ICP-AAS measurement was described that used a hollow cathode lamp as a radiation source and an ICP as an atomization cell [5]. A later work reported detection limits for ICP-AAS for determination of Ag, Al, Ca, Cu, Mo, Ta and V at about ppm levels using a short plasma torch as an absorption cell [6]. Based on theoretical considerations and calculations, Magyar, et al., suggested ICP might not be an ideal source for AAS measurement [7]. The following reasons were identified: 1) high plasma gas flow rate required for maintaining the ICP dilutes the concentration of analyte atoms, resulting in a short residence time of analyte in the plasma; 2) absorption path length in an ICP is relatively short, and not beneficial for AAS measurement; and 3) high temperatures in the ICP source favor the production of excited and ionized species while AAS needs ground level populations. Therefore, subsequent research on ICP-AAS has mainly focused on fundamental studies and plasma diagnostics [8–10].
Although CRDS has rapidly gained popularity in the molecular spectroscopy community, there are few reports exploring atomic absorption with CRDS [11–13]. Thus far, the only published research has used inductively coupled plasma as an atomization cell for CRDS measurement [11–12]. This exploratory research showed very promising results to adapt CRDS for atomic absorption measurement in plasma sources. However, using a conventional ICP as an atomization cell for AAS measurement has some obvious limitations [7]. It is widely believed in the art that the ICP can be a poor source for ground state neutral species because most of the analyte atoms are either excited or ionized at conventional ICP's. With such knowledge in mind, most recent work on ICP-CRDS has lowered the ICP power in combination with a modification to the ICP torch design so that ground state populations of analytes can be significantly enhanced, allowing improved detection limits to be achieved using cavity ring-down spectroscopy [12]. However, a lowered operating power makes the ICP plasma source less robust or even fragile.
Microwave induced plasma (MIP) is a powerful alternative source for elemental determination and has been extensively used in analytical atomic spectrometry [14]. Compared with other types of plasma sources, the MIP offers some attractive characteristics, such as its unique features of high excitation efficiency for metal and nonmetal elements; capability of working with various gases; simplicity; and low cost for instrumentation and maintenance. In addition, microwave plasmas can be sustained at fairly low power and low gas flow rate, making them a desirable source for absorption measurement [15]. There have been publications reporting the use of microwave plasma sources as atomization cells for conventional AAS measurement with hollow cathode lamps (Hcl) [16–19] to pursue sensitivity, including designing high efficiency desolvation devices to remove water vapor loading [18], designing various plasma discharges for better absorption measurement [15], adapting different sampling device [17], and regulating the plasma gas flow system. With conventional lamps as light source, MIP-AAS gives about two to three orders of magnitude better results than ICP-AAS does [15].
In spite of some minor advances made in various related technologies in recent years, it remains the rule that the limitations of conventional elemental and isotope measurement and diagnosis is still bound by obvious sensitivity limitations.
By the present invention, these long-standing limitations have been addressed to provide a system that gives high sensitivity and capability for elemental and hyperfine structure measurements.